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In my opinion "description of trajectory" is not a topic of physics it is a topic of dynamic geometry. (Whereas explaining the mechanism which causes an object to follow a particular trajectory IS a matter of physics).

To say that "B goes around C" is to describe a trajectory in space and time. A trajectory may be described by graphical means e.g. a circle, an ellipse, a helix. But all such graphical presentations of a trajectory are subjective i.e. they depend on the frame of the observer. An observer attached to C will observe that B goes around C. Whereas an observer attached to B will observe that C goes around B.

In a Euclidean system of description a particular object trajectory can be described absolutely by relating the displacements (distance and direction) of the object (at various moments in time) relative to one or more reference objects (whose trajectories are themselves known ... relative to some useful standard).

If you sit in an office chair and someone spins it around you will see the walls of the office move around you. I contend that it can be acceptable and useful to say that "the office moves around you". Likewise it is misleading to say categorically that "the office DOES NOT move around you". Any description of movement (motion) relates the positions of at least two objects. This applies to both linear and non-linear patterns of motion. Physicists may choose to describe, measure and account for the sensations and motions which you experience by choosing particular frames of reference because they are more simple or more useful. But this does not dictate how you choose to describe the dynamic geometry of your experience.

Therefore the following descriptions of dynamic geometry are all acceptable:- "the Earth moves around the Sun" "the Sun moves around the Earth". "the Sun and the Earth each move around their mutual barycentre".

In my opinion "description of trajectory" is not a topic of physics it is a topic of kinematics (geometry of motion, see http://en.wikipedia.org/wiki/Kinematics). Whereas explaining the mechanism which causes an object to follow a particular trajectory IS a matter for physics.

To say that "B goes around C" is to describe a trajectory in space and time. A trajectory may be described by graphical means e.g. a circle, an ellipse, a helix. But all such graphical presentations of a trajectory are subjective i.e. they depend on the frame of the observer. An observer attached to C will observe that B goes around C. Whereas an observer attached to B will observe that C goes around B.

In a Euclidean system of description a particular object trajectory can be described absolutely by relating the spatial displacements (distance and direction) of the object (at various moments in time) relative to one or more reference objects (whose trajectories are themselves known ... relative to some useful standard).

If you sit in an office chair and someone spins it around you will see the walls of the office move around you. I contend that it can be acceptable and useful to say that "the office moves around you". Likewise it is misleading to say categorically that "the office DOES NOT move around you". Any description of movement (motion) relates the positions of at least two objects. This applies to both linear and non-linear patterns of motion. Physicists may choose to describe, measure and account for the sensations and motions which you experience by choosing particular frames of reference because they are more simple or more useful. But this does not dictate how you choose to describe the dynamic geometry of your experience.

Therefore the following descriptions of dynamic geometry are all acceptable and potentially useful and potentially ambiguous:- "the Earth moves around the Sun" "the Sun moves around the Earth". "the Sun and the Earth each move around their mutual barycentre".

In my opinion "description of trajectory" is not a topic of physics it is a topic of dynamic geometry. (Whereas explaining the mechanism which causes an object to follow a particular trajectory is a matter of physics).

To say that "A goes around B" is to describe a trajectory in space and time. A trajectory may be described by graphical means e.g. a circle, an ellipse, a helix. But all such graphical presentations of a trajectory are subjective i.e. they depend on the frame of the observer. An observer attached to B will observe that A goes around B. Whereas an observer attached to A will observe that B goes around A.

In a Euclidean system of description a particular object trajectory can be described absolutely by relating the displacements (distance and direction) of the object (at various moments in time) relative to one or more reference objects (whose trajectories are themselves known ... relative to some useful standard).

If you sit in an office chair and someone spins it around you will see the walls of the office move around you. I contend that it can be acceptable and useful to say that "the office moves around you". I contend that it is misleading to say categorically that "the office does not move around you". How physicists choose to measure and account for the sensations and motions do not constrain how you choose to describe the dynamic geometry of your experience.

Therefore the following descriptions of dynamic geometry are all acceptable:- "the Earth moves around the Sun" "the Sun moves around the Earth". "the Sun and the Earth each move around their mutual barycentre".

In my opinion "description of trajectory" is not a topic of physics it is a topic of dynamic geometry. (Whereas explaining the mechanism which causes an object to follow a particular trajectory IS a matter of physics).

To say that "B goes around C" is to describe a trajectory in space and time. A trajectory may be described by graphical means e.g. a circle, an ellipse, a helix. But all such graphical presentations of a trajectory are subjective i.e. they depend on the frame of the observer. An observer attached to C will observe that B goes around C. Whereas an observer attached to B will observe that C goes around B.

In a Euclidean system of description a particular object trajectory can be described absolutely by relating the displacements (distance and direction) of the object (at various moments in time) relative to one or more reference objects (whose trajectories are themselves known ... relative to some useful standard).

If you sit in an office chair and someone spins it around you will see the walls of the office move around you. I contend that it can be acceptable and useful to say that "the office moves around you". Likewise it is misleading to say categorically that "the office DOES NOT move around you". Any description of movement (motion) relates the positions of at least two objects. This applies to both linear and non-linear patterns of motion. Physicists may choose to describe, measure and account for the sensations and motions which you experience by choosing particular frames of reference because they are more simple or more useful. But this does not dictate how you choose to describe the dynamic geometry of your experience.

Therefore the following descriptions of dynamic geometry are all acceptable:- "the Earth moves around the Sun" "the Sun moves around the Earth". "the Sun and the Earth each move around their mutual barycentre".

In my opinion "description of trajectory" is not a topic of physics it is a topic of dynamic geometry. (Whereas explaining the mechanism which causes an object to follow a particular trajectory is a matter of physics).

To say that "A goes around B" is to describe a trajectory in space and time. A trajectory may be described by graphical means e.g. a circle, an ellipse, a helix. But all such graphical presentations of a trajectory are subjective i.e. they depend on the frame of the observer. An observer attached to B will observe that A goes around B. Whereas an observer attached to A will observe that B goes around A.

In Euclidean and Galilean systems of description a particular object trajectory can be described absolutely by relating the displacements (distance and direction) of the object (at various moments in time) relative to one or more reference objects (whose trajectories are themselves known ... relative to some useful standard).

If you sit in an office chair and someone spins it around you will see the walls of the office move around you. I contend that it can be acceptable and useful to say that "the office moves around you". I contend that it is misleading to say categorically that "the office does not move around you". How physicists choose to measure and account for the sensations and motions do not constrain how you choose to describe the dynamic geometry of your experience.

Therefore the following descriptions of dynamic geometry are all acceptable:- "the Earth moves around the Sun" "the Sun moves around the Earth". "the Sun and the Earth each move around their mutual barycentre".

In my opinion "description of trajectory" is not a topic of physics it is a topic of dynamic geometry. (Whereas explaining the mechanism which causes an object to follow a particular trajectory is a matter of physics).

To say that "A goes around B" is to describe a trajectory in space and time. A trajectory may be described by graphical means e.g. a circle, an ellipse, a helix. But all such graphical presentations of a trajectory are subjective i.e. they depend on the frame of the observer. An observer attached to B will observe that A goes around B. Whereas an observer attached to A will observe that B goes around A.

In a Euclidean system of description a particular object trajectory can be described absolutely by relating the displacements (distance and direction) of the object (at various moments in time) relative to one or more reference objects (whose trajectories are themselves known ... relative to some useful standard).

If you sit in an office chair and someone spins it around you will see the walls of the office move around you. I contend that it can be acceptable and useful to say that "the office moves around you". I contend that it is misleading to say categorically that "the office does not move around you". How physicists choose to measure and account for the sensations and motions do not constrain how you choose to describe the dynamic geometry of your experience.

Therefore the following descriptions of dynamic geometry are all acceptable:- "the Earth moves around the Sun" "the Sun moves around the Earth". "the Sun and the Earth each move around their mutual barycentre".